"For most people they might just be browsing the web and suddenly they don't know why this thing will pop up in their face, telling them they've got 309 infections on their computer, it will change their desktop wallpaper, change their screensaver to fake 'blue screens of death'," Stewart said.

"It goes to a lot of trouble to try and scare people into thinking they have a massive infection on their computer and they need to pay money to this software vendor to get it cleaned."

Stewart said the pop-ups keep re-appearing until the user buys the anti-virus program, which looks much like real security software. Once they do, another pop-up says the computer is being scanned and all problems are fixed, but "everything it just scanned for is fake so it didn't really do anything".